Photosynthesis: The Light Reactions PDF

# Oxygenic Photosynthesis Evolved Around 2.4 Billion Years Ago This graph shows the evolution of oxygen levels in the Earth's atmosphere over time. - **Time (billions of years ago):** 4.6, 3.6, 2.6, 1.6, 0.6, Present day - **Oxygen levels in atmosphere (%):** 0, 10, 20 **Key Events:** - **4.6 BY...

# Oxygenic Photosynthesis Evolved Around 2.4 Billion Years Ago This graph shows the evolution of oxygen levels in the Earth's atmosphere over time. - **Time (billions of years ago):** 4.6, 3.6, 2.6, 1.6, 0.6, Present day - **Oxygen levels in atmosphere (%):** 0, 10, 20 **Key Events:** - **4.6 BYA:** Formation of the earth - **3.6 BYA:** Formation of oceans and continents - **3.6 BYA:** First living cells - **2.6 BYA:** First photosynthetic cells (bacteria) - **2.4 BYA:** First water-splitting photosynthesis releases O2 - **2.4 BYA:** Aerobic (oxygen-using) respiration becomes widespread - **2.4 BYA:** Origin of eukaryotic photosynthetic cells - **1.6 BYA:** Start of rapid O2 accumulation - **0.6 BYA:** First vertebrates - **Present day:** First multicellular plants, fungi, and animals **Early Photosynthetic Organisms:** - Early photosynthetic organisms probably used **H2S** as an electron source. - **Earth's atmosphere was anaerobic.** **Oxygenic Photosynthesis:** - **Oxygenic photosynthesis - using H2O as the electron source evolved around 2.4 BYA.** - **Atmospheric oxygen concentration rose.** - This **necessitated the evolution of oxidative phosphorylation.** # Photosynthesis: The Light Reactions - Photosynthesis uses light energy to boost electrons from a low-energy to a high-energy excited state. - These electrons are used to create a proton-motive force that powers the synthesis of **ATP**. - The high-energy electrons are also used to form **NADPH**, biosynthetic reducing power. - The reactions powered by sunlight are called the **light reactions**. The diagram shows a green circle surrounded by a larger circle, both with an arrow on each side showing the flow of electrons from left to right, using light. - **Photosystem II:** The light reactions begin here with the splitting of **water** molecules. The electrons released travel through a chain of carriers, moving from lower to higher energy and releasing energy along the way to form **ATP** and a proton gradient. - **Proton Gradient:** The proton gradient builds up in the thylakoid lumen. - **Photosystem I:** Light energy is absorbed here. - **Reducing Power:** The electrons end up on **NADPH** forming and increasing reducing power. <start_of_image> Diagrams show the different steps of the process, explaining: - **Absorption of light**: Absorption of light excites electrons in a pigment. - **Resonance Energy Transfer**: When an electron falls to a lower energy level, energy is released and transferred to a nearby molecule. - **Photoinduced Charge Separation**: Electron transfer results in a charge separation, where the initial molecule is now positively charged and the molecule that accepted the electron is negatively charged. # Photosynthesis in Eukaryotes Takes Place in Chloroplasts - The chloroplast is a **double-membrane organelle**. - The **inner membrane** surrounds a space called the **stroma**, which is the site of glucose synthesis from CO2 and H2O using ATP and NADPH formed in the light reactions. - In the stroma are membranous sacs called **thylakoid membranes**. Thylakoid membranes are the location of the light reactions of photosynthesis. The diagram shows a cross-section of a chloroplast. - **Outer membrane:** The outer membrane is the outermost layer of the chloroplast. - **Inner membrane:** The inner membrane is located inside the outer membrane. - **Stroma:** This is the space between the inner and outer membranes. - **Thylakoid membranes:** These membranes are located inside the stroma. - **Thylakoid lumen:** This is the space inside the thylakoid membranes. # Chlorophyll is the Primary Light Acceptor in Most Photosynthetic Systems - The primary photoreceptor in the chloroplasts of green plants is **chlorophyll**, a substituted tetrapyrrole bound to a large hydrophobic 20-carbon alcohol called phytol. - Chlorophyll is a highly conjugated molecule. - The nitrogens of the pyrrole are coordinated to **magnesium**. - Chlorophyll *a* and *b* have different absorption spectra. The diagram shows the chemical structures of chlorophyll *a* and *b*. - There is a table with a maximum wavelength for chlorophyll *a* and *b*. The maximum wavelengths are 420 nm, 670nm for chlorophyll *a* and 460 nm, 647 nm for chlorophyll *b*. - **Absorption of light by chlorophyll results in charge separation**. This diagram shows the reaction center of Photosystem I. - Light energy is absorbed by chlorophyll to give an excited state electron. - The excited state electron is then donated to an acceptor molecule. - This acceptor molecule now has reducing power. # Light Harvesting Molecules and the Photosynthetic Reaction Centers are Embedded in the Thylakoid Membranes The diagram shows a cross-section of a thylakoid membrane. - **Antenna chlorophylls:** These are light-harvesting molecules that absorb the light energy and transfer it to the reaction center. - **Carotenoids and other accessory pigments:** These pigments also absorb light energy and transfer it to antenna chlorophylls. - **Reaction center:** The reaction center is where the light energy is converted into chemical energy. - **The efficiency of photosynthesis is increased by the use of antenna molecules**. - **These pigments absorb light energy, while differing from chlorophyll in the reaction centers**. - **The antennae pigments funnel energy to the reaction center using resonance energy transfer**. # Photosynthesis in Green Plants Consists of Two Photosystems. The diagram shows a cross-section of a thylakoid membrane with two photosystems. - **Photosystem I:** This photosystem produces biosynthetic reducing power in the form of NADPH. - **Photosystem II:** This photosystem replaces electrons lost by Photosystem I and generates a proton gradient that is used to synthesize ATP. - The missing electrons in Photosystem II are replaced by the **photolysis of water.** # Photosystem I is a Massive, Membrane-Spanning Complex of a Dozen Proteins and Hundreds of Cofactors, Including Chlorophyll, Quinones, and Iron-Sulfur Clusters The diagram shows a 3D model of Photosystem I. - The heart of Photosystem I is an electron transfer chain, a chain of chlorophyll, phylloquinone, and three iron-sulfur clusters. - The final electron acceptor is an iron-sulfur protein ferredoxin. # Photosystem I Seen From Above The diagram shows a top view of Photosystem I. - **The reaction centers are surrounded by additional chlorophyll and carotenoid molecules which harvest and funnel light energy to the centers.** # The Special Pair in Photosystem I is P700 The diagram shows a simplified schematic of Photosystem I. - The reaction center in Photosystem I is called P700. - It consists of a pair of chlorophyll molecules with absorption maxima around 700 nm. - The electrons from excited P700 flow down an electron-transport chain to the iron-sulfur protein ferredoxin. - **Ferredoxin-NADP+ reductase transfers electrons from ferredoxin to form NADPH, biosynthetic reducing power.** # Ferredoxin-NADP+ Reductase Mediates Between the Obligatory One-Electron Transfer From Ferredoxin and The Obligatory Two-Electron Transfer to NADP+ The diagram shows a simplified schematic of the ferredoxin-NADP+ reductase enzyme. - **Notice: NADPH is made on the stromal side of the thylakoid membrane. H+ are also pumped across the thylakoid membrane into the thylakoid lumen as a result of electron transport through PS I.** # How Do We Replenish The Electrons Given Up By P700+? # Photosystem II Transfers Electrons to Photosystem I and Generates a Proton Gradient for ATP Synthesis. The diagram shows a 3D model of Photosystem II. - Photosystem II: D2 and D1 proteins act as a scaffold for a trans-membrane assembly of more than 20 cofactors and other proteins - **The special pair of chlorophyll molecules in Photosystem II is called P680°.** - **P680° transfers electrons to Photosystem I.** - The electron transport chain includes pheophytin, plastoquinone, and the cytochrome *b6f* complex. # The Special Pair in Photosystem II is P680 - The diagram shows the chemical structure of plastoquinone in its oxidized and reduced forms. # Cytochrome *b6f* Links Photosystem II to Photosystem I - The diagram shows a cross-section of a thylakoid membrane with the cytochrome *b6f* complex. - Cytochrome *b6f* releases protons from plastoquinol into the thylakoid lumen and pumps protons from the stroma into the lumen. - **The mechanism is similar to the Q cycle of Complex III in the electron-transport chain of cellular respiration.** - **Reduced plastocyanin donates electrons back to Photosystem I.** # The Water-Oxidizing Complex (WOC) of PS II Replenishes the Electron Lost by the Special Pair - The diagram shows a 3D model of the water-oxidizing complex (WOC) of Photosystem II. - The lower half of the reaction center of PS II contains the special pair. - Excitation of the special pair leads to electron transport, leaving the original chlorophyll without an electron. - The upper half of the reaction center replaces this electron with a low-energy electron from water. - The oxygen-evolving center strips an electron from water and passes it to a tyrosine amino acid. - The tyrosine amino acid delivers an electron to the chlorophyll. - This reaction, the photolysis of water, occurs at the water-oxidizing complex (WOC) of Photosystem II. - Manganese is a key component of the WOC. # The Electron Flow From H2O to NADP+ is Called the Z-Scheme of Photosynthesis - The diagram shows a simplified schematic of the Z-scheme of photosynthesis. - **The Z-scheme shows the electron flow from H2O to NADP+, which generates a proton-motive force.** # Some Organisms Use Electron Donors Other Than Water - The table lists the major groups of photosynthetic bacteria and their electron donors. # The Flow of Electrons From H2O to NADP+ Generates a Proton-Motive Force - **In chloroplasts, most of the energy of the proton-motive force consists of the proton gradient with the membrane potential contributing little energy.** - **In chloroplasts, electroneutrality is maintained because Mg2+ moves into the stroma when 2 H+ are pumped from the stroma into the thylakoid lumen. In mitochondria, the membrane potential also contributes to the proton-motive force.** # The ATP Synthase of the Chloroplast is Also Called the CF1-CF0 Complex, Where C Stands for Chloroplast - **The chloroplast CF1-CF0 complex is very similar to the mitochondrial F1-Fo complex.** - **Newly synthesized ATP is released into the stroma, where it is used in carbohydrate synthesis.** # A Summary of the Light Reactions of Photosynthesis - The diagram shows a simplified schematic of the light reactions of photosynthesis. - **Photosystem II**: splits water molecules. - **Cytochrome *b6f* complex**: uses the electrons from the water split to create the proton gradient. - **Photosystem I**: uses light energy to create NADPH. It does not split water. - **ATP Synthase**: uses the proton gradient to create ATP. # Fig. 1 High-Resolution Cryo-EM Map of the Spinach Chloroplast cF1Fo ATP Synthase - The diagram shows a high-resolution cryo-EM map of the spinach chloroplast cF1Fo ATP synthase. - **Note the orange subunit – during photosynthesis, Photosystem I generates reduced ferredoxin.** - **Reduced ferredoxin reduces a cystine bridge in this regulatory subunit, activating the ATP synthase activity.** - **In the absence of light, proton motive force drops. Reduced ferredoxin concentration drops. Cystine bridge reforms on the regulatory subunit, inactivating the ATP synthase.** # The Activity of Chloroplast ATP Synthase Is Regulated By The Availability of Reducing Agents and The Proton Motive Force - **ATP synthase is sensitive to redox conditions in the chloroplast. For maximal activity, a disulfide bond in the y subunit of ATP synthase must be reduced to two cysteines.** - The reductant is thioredoxin, which is formed from ferredoxin generated in Photosystem I. - **The ε subunit of ATP synthetase is sensitive to increases in the proton-motive force, resulting in a conformation which facilitates the reduction of the disulfide bond in the y subunit of ATP synthase, again up-regulating ATP synthesis.** # Cyclic Photophosphorylation Diverts Electron Flow to the Production of ATP Instead of NADPH - **If the NADPH needs are adequate, cyclic electron flow generates ATP without forming NADPH, or producing O2.** - The electrons of Photosystem I flow from ferredoxin through cytochrome *b6f* to plastocyanin and then return to P700. - The protons pumped by cytochrome *b6f* are used to synthesize ATP. # Eight Photons Yield One O2, Two NADPH, and Three ATP - **Eight photons result in the pumping of 12H+ into the thylakoid lumen.** - 12H+ passing through the ATP synthase complete one full rotation of CF1 to yield 3 ATP - **In cyclic photophosphorylation, two photons yield one molecule of ATP but not NADPH.** # The Components of Photosynthesis Are Highly Organized - Thylakoid membranes are organized into stacked and unstacked regions. - Photosystem II is located in the stacked regions. - Photosystem I and ATP synthase are in unstacked regions, allowing their products—NADPH and ATP, respectively—ready access to the carbohydrate-synthesizing enzymes of the stroma. - Photosystem II can function in stacked regions as it only requires access to water and plastoquinone. The diagram shows a cross-section of a chloroplast with the different components of photosynthesis. # Many Herbicides Inhibit the Light Reactions of Photosynthesis - Diuron and atrazine are herbicides that inhibit Photosystem II. - Paraquat inhibits Photosystem I and generates reactive oxygen species. The diagram shows the chemical structures of diuron, atrazine, and paraquat. # You Need To Know - How is light energy turned into reducing power? - What is the site of photosynthesis? - Photosystem I: produces NADPH in the stromal space, gets electrons from Photosystem II via cytochrome B6f and plastocyanin. - Photosystem II: generates a proton gradient, gets electrons from water, generating oxygen. - Cytochrome B6f: contributes to proton gradient; mediate electron flow between PS II and PS I. - Cyclic photophosphorylation—makes more ATP when NADPH is plentiful. - Reduced ferredoxin from PS I upregulates ATP synthetase via reduced thoredoxin. - Proton gradient upregulates ATP synthetase. - Protons pump OUT of thylakoid lumen via ATP synthetase, synthesizing ATP in the stromal space. - PS I and ATP synthase concentrated in unstacked regions of the thylakoid membranes. - PSII concentrated in stacked regions—doesn't make products that need to get into the stromal space.